Shielded gate trench (SGT) MOSFET devices and manufacturing processes

ABSTRACT

This invention discloses a semiconductor power device that includes a plurality of power transistor cells surrounded by a trench opened in a semiconductor substrate. At least one of the cells constituting an active cell has a source region disposed next to a trenched gate electrically connecting to a gate pad and surrounding the cell. The trenched gate further has a bottom-shielding electrode filled with a gate material disposed below and insulated from the trenched gate. At least one of the cells constituting a source-contacting cell surrounded by the trench with a portion functioning as a source connecting trench is filled with the gate material for electrically connecting between the bottom-shielding electrode and a source metal disposed directly on top of the source connecting trench. The semiconductor power device further includes an insulation protective layer disposed on top of the semiconductor power device having a plurality of source openings on top of the source region and the source connecting trench provided for electrically connecting to the source metal and at least a gate opening provided for electrically connecting the gate pad to the trenched gate.

This Patent Application is a Divisional Application and claims thePriority Date of a application Ser. No. 11/356,944 filed on Feb. 17,2006 now issued into U.S. Pat. No. 7,633,119 and a application Ser. No.12/653,355 filed on Dec. 11, 2009 now U.S. Pat. No. 7,936,011 by commonInventors of this Application. The Disclosures made in the patentapplication Ser. Nos. 11/356,944 and 12/653,355 are hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the semiconductor power devices. Moreparticularly, this invention relates to an improved and novelmanufacturing process and device configuration for providing the MOSFETdevice with shielded trench gates (STG) for providing high frequencypower switching device.

2. Description of the Prior Art

Conventional technologies for reducing the gate to drain capacitance Cgdin a power semiconductor device are still confronted with technicallimitations and difficulties. As there are growing demands for highfrequency switch power devices, an urgent need exists to provideeffective solutions to achieve the purpose of resolving these technicaldifficulties and limitations. For power transistors including MOSFET andIGBT, a new device configuration and manufacturing process are necessaryto reduce the speed-limiting capacitance between the gate and the drainof these switching power devices.

Baliga discloses in U.S. Pat. No. 5,998,833 a DMOS cell as shown in FIG.1A. A source electrode is placed underneath the trenched gate to reducethe gate-to-drain capacitance. The gate of the DMOS cell is divided intotwo segments. The gate-to-drain capacitance is reduced because thecontributions to capacitance from the gate-drain overlapping areas areeliminated.

In U.S. Pat. No. 6,690,062, a MOSFET device as shown in FIG. 1B isdisclosed where the switching behavior of a transistor configuration isimproved by providing a shielding electrode in an edge region. Theshielding electrode surrounds at least sections of an active cell array.There is a capacitance between an edge gate structure and a drain zone.The shielding electrode located in the edge region reduces thecapacitance between an edge gate structure and a drain zone hencereduces the gate-drain capacitance C_(GD) of the transistor.

In U.S. Pat. No. 6,891,223, Krumrey et al. disclose a transistor thatincludes transistor cells disposed along trenches in a semiconductorsubstrate with two or more electrode structures disposed in thetrenches. Furthermore, metallization structures are disposed above thesubstrate surface as shown in FIG. 1C. The trenches extend into aninactive edge region of the transistor. An electrical connection betweenthe electrode structures and corresponding metallization are establishedin the edge regions.

The above patented-disclosures including transistor configurations stillhave a common difficulty. The source electrode disposed on the trenchbottom is connected to the source voltage through an edge area of thesemiconductor power device. This inevitably increases the sourceelectrode resistance. Furthermore, the extra masks needed to create suchconnection also increase the cost of manufacturing.

Therefore, a need still exists in the art of power semiconductor devicedesign and manufacture to provide new manufacturing method and deviceconfiguration in forming the power devices such that the above discussedproblems and limitations can be resolved.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide a new andimproved semiconductor power device implemented with the shielded gatetrench (SGT) structure that has the bottom-shielding electrode providedwith improved connection more directly to the source voltage.Specifically, a macro-cell layout approach is disclosed. In themacro-cell, a trench filled with conductive material such as dopedpolysilicon is employed to electrically connect the bottom-shieldingelectrode of the SGT structure directly to the source metal. Theabove-discussed problems and difficulties of the conventionconfigurations with source voltage connections via a peripheral portionof the device are therefore resolved.

Specifically, it is an aspect of the present invention to provideimproved SGT device configuration and manufacturing method to reduce thegate to drain capacitance. A new method for manufacturing asemiconductor power device is disclosed. The method includes a step ofopening a trench in a substrate for surrounding a plurality of powertransistor cells and filling the trench with a gate material. The methodfurther includes a step of applying a time etch for etching back thegate material from a selected portion of the trench followed by coveringa bottom portion of the gate material in the selected portion of thetrench with an insulation layer to form a bottom-shielding electrodewhile keeping the gate material in a remainder portion of the trench asa source connecting electrode to maintain direct electrical connectionto the bottom-shielding electrode. This method of manufacturingsemiconductor power device provides direct electrical connections of theSGT structure disposed underneath the trenched gate to the source metalvia a plurality of trenched “source connecting electrodes” with thesesource connecting trenches filled with the gate material, e.g., the gatepolysilicon, servicing as interconnection for the SGT to the sourcemetal. This direct electrical connection is provided by a next step offorming an electrical connection from the gate material in the remainderportion of the trench to a source metal. Furthermore, the methodincludes a step of filling a selected portion of a trench with the gatematerial and forming an electrical connection for connecting thetrenched gate to a gate pad. In the manufacturing processes, there isfurther step of controlling the time etch in removing the gate materialfrom a top portion of the selected portion of the trench for controllinga depth of a trenched gate of the semiconductor power device. The methodfurther includes a step of forming an insulation layer for covering atop surface of the semiconductor power device and opening a plurality ofsource contact openings on top of the remainder portion of the trench toform source contact to direct contact the gate material in the remainderportion of the trench for electrically connecting to thebottom-shielding electrode. Furthermore, the method further includes astep of forming an insulation layer for covering a top surface of thesemiconductor power device and opening at least a gate contact openingfor providing a gate pad to electrically connecting to the gate materialin the trenched gate in the selected portion of the trench.

Briefly in a preferred embodiment this invention discloses a trenchedsemiconductor power device. The trenched semiconductor power deviceincludes a plurality of interconnected trenches formed on asemiconductor substrate. At least one of the interconnected trenchesconstitutes a shielded gate trench (SGT) for the semiconductor powerdevice. The SGT includes a trenched gate disposed on an upper portion ofthe SGT and a bottom-shielding electrode disposed on a bottom portion ofthe SGT insulated from the trenched gate. At least one of theinterconnected trenches constitutes a source-connecting trench filledwith a conductive trench-filling material electrically connected to thebottom-shielding electrode of the SGT for electrically connecting to asource metal disposed on top of the source-connecting trench

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are cross sectional views of trenched MOSFET devicesdisclosed in patented disclosures for reducing gate-drain capacitance.

FIGS. 2A is a top view and 2B to 2D are three cross sectional views of atrenched MOSFET device implemented with improved configuration of thisinvention.

FIGS. 3A to 3L are a serial cross sectional views for describing amanufacturing process to provide a trenched MOSFET device as shown inFIG. 2.

FIGS. 3M to 3O are a serial cross sectional views for describing anothermanufacturing process to provide a trenched MOSFET device as shown inFIG. 2.

FIG. 4 is a side cross sectional view of another embodiment of thisinvention with specially configured termination area of a MOSFET device

FIG. 5 shows another embodiment of this invention of a MOSFET devicewith specially configured SGT trenched gate to further reduce the gateto drain capacitance.

DETAILED DESCRIPTION OF THE METHOD

Referring to FIGS. 2A to 2D for a top view and three cross sectionalviews respectively of a trenched MOSFET device 100 of this invention. Asshown in FIG. 2B, the trenched MOSFET device 100 is supported on asubstrate 105 formed with an epitaxial layer 110. The trenched MOSFETdevice 100 includes a shielded gate trench (SGT) structure. The SGTstructure includes a bottom-shielding electrode 130 insulated from anddisposed below a trenched gate 150. The bottom SGT structure 130 filledwith a polysilicon therein is provided to shield the trenched gate 150from the drain disposed below trench bottom. The bottom SGT structure130 is insulated from the drain region by a dielectric layer 113. Aninsulation layer 120 separated the bottom-shielding electrode 130 fromthe trenched gate 150. The trenched gate 150 includes polysiliconfilling in a trench surrounded with a gate insulation layer 155 coveringthe trench walls. A body region 160 that is doped with a dopant ofsecond conductivity type, e.g., P-type dopant, extends between thetrenched gates 150. The P-body regions 160 encompassing a source region170 doped with the dopant of first conductivity, e.g., N+ dopant. Thesource regions 170 are formed near the top surface of the epitaxiallayer surrounding the trenched gates 150. On the top surface of thesemiconductor substrate is also an insulation layer 180. The contactopenings 185 and 195 are opened through the insulation layer 180 tocontact the source metal layer 190. As shown in FIGS. 2C-2D, thebottom-shielding electrode 130 is electrically connected to the sourcemetal 190 through the trenched source-connecting electrode 140. Thetrenched source-connecting electrode 140 is electrically connected tothe bottom-shielding electrode 130 through the interconnected trenchesthat extend between the MOSFET cells. The trenched source connectingelectrode 140 may be extrude beyond the top surface of body region 160and source region 170 as shown in FIG. 2C to increase the area ofcontact.

FIG. 2A shows a macro cell layout of the device wherein each active cellhas a square layout defined by surrounding trenches formed as trenchedgates 150 with bottom-shielding electrode 130 functioning as the SGTstructure. In region 195 where two trenches intersect, a trenched sourceconnecting electrode is formed to electrically connect tobottom-shielding electrode 130. This region 195 may be extended beyondthe intersection region so that the trenched source connecting electrode140 would extend into a portion of the trenches. Alternatively it isalso possible to form the source connecting electrode 140 in a region195 where trenches do not intersect. In addition to squares as shown inFIG. 2A, other kind of polygons, such as triangle, rectangular andhexagonal may also be implemented. Each macro cell as shown in FIG. 2Aincludes a plurality of active cells 115 and at least a region 125. Theactive cells 115 is defined and surrounded by trenched gates 150 whileinside the area 125, the trench 140 is filled with gate filling materialto electrically connect the bottom-shielding electrode 130 to the sourcecontact metal. FIG. 2C shows the boundary lines of the area 125 and theactive cells 115. Furthermore, the bottom-shielding electrode 130 of theSGT structure is shown as connected to the trenched source-connectingelectrode 140 through interconnecting trenches 130 (in the active cellarea 115) and also trench 140 (in the source-contact trench area 125).Interconnections of these trenches are provided through a thirddimension and via trenches behind and before the cross sectional surfaceas shown in FIG. 2D.

According FIGS. 2A to 2D and above descriptions, this inventiondiscloses a trenched semiconductor power device. The trenchedsemiconductor power device includes a plurality of interconnectedtrenches form on a semiconductor substrate. At least one of theinterconnected trenches constitutes a shielded gate trench (SGT) for thesemiconductor power device. The SGT includes a trenched gate disposed onan upper portion of the SGT and a bottom-shielding electrode disposed ona bottom portion of the SGT insulated from the trenched gate. At leastone portion of the interconnected trenches constitutes asource-connecting trench filled with a conductive trench-fillingmaterial electrically connected to the bottom-shielding electrode of theSGT for electrically connecting to a source metal disposed on top of thesource-connecting trench.

Referring to FIGS. 3A to 3L for a serial of side cross sectional viewsto illustrate the fabrication steps of a MOSFET device as that shown inFIGS. 2A to 2D. In FIG. 3A, a trench mask 208 is applied as first maskto create an oxide hard mask 206 and then removed. Referring to FIG. 3B,a trench etch process is carried out to open a plurality of trenches 209in an epitaxial layer 210 supported on a substrate 205. The net depth asrequired for both electrodes and the targeted oxide thickness determinesthe trench depth. In FIG. 3C, a sacrificial oxidation is performedfollowed by an oxide etch to remove the damaged surface on the trenchwall to smooth the sidewalls. Then a gate oxidation is performed to growa gate oxide layer 215. A thick oxide layer 215 is grown to a thicknessbased on device optimization for low Rds and high breakdown voltage. Athicker oxide layer 215 here reduces the silicon surface electric field,allowing the use of higher doping and leading to lower Rds for the samebreakdown rating.

In FIG. 3D, a source polysilicon layer 220 is deposited into thetrenches 209. In FIG. 3E, a blanket polysilicon etch back is performedto etch back the source polysilicon layer 220. The source polysiliconlayer 220 is etched back without a mask until it is just below the topsurface of the oxide. In FIG. 3F, a second mask, i.e., a sourcepolysilicon mask 222, is applied to cover a portion of sourcepolysilicon layer 220 inside designated source contact trenches. Thenthe source polysilicon layer 220 is etched back to remove the upperportion inside the trenches designated for trenched gate. The sourcepolysilicon 220 is etched to a target depth using a timed etch-backprocess. The source polysilicon mask 222 is removed. The thick oxide isstripped using a wet etch, until the top surface and sidewalls are clearin the area not encapsulated by the source polysilicon. Care is taken tonot excessively over etch this oxide within the lower portion of trench.In FIG. 3G, a thin gate oxide layer 225 is formed to cover the upperportion of trench wall and the top surface of the remaining sourcepolysilicon layer 220 to form the bottom-shielding electrode. A thinoxide on the gate trench sidewall provides the advantage of reducinggate threshold voltage. The gate oxidation process grows a thicker oxideover all the exposed poly regions, due to the well known enhancedoxidation in doped polysilicon. This thicker oxide surrounding thebottom SGT structure has the advantage of improving the breakdownvoltage. In FIG. 3H, a gate polysilicon layer 230 is deposited into thegate trenches and etched back to form the trenched gate. This gatepolysilicon layer 230 is simply etched back using no mask, until it lieseverywhere just below the surface of the top surface oxide.

In FIG. 3I, a body dopant implant to form a plurality of body dopantregions 235 is carried out by employing a body mask (not shown). Thisbody mask excludes the body region from specific locations in thetermination area leading to the formation of guard ring type terminationstructures. The termination area structure will allow the device toblock high voltages. In FIG. 3J, the body mask is removed followed by abody diffusion to form the body regions 235. The body drive diffuses thedopant to the desired depth that is no deeper than the upper gateelectrode. Then a fourth mask, i.e., the photoresist as source mask 237,is applied to carry out a source dopant implant to form a plurality ofsource dopant regions 240. A local oxide thinning is performed beforethe source is implanted. In FIG. 3K, the photoresist layer 237 isremoved, followed by applying an elevated temperature to diffuse thesource regions 240. After a source drive, the LTO layer 245 and BPSGlayer 250 are deposited. Then, a BPSG flow process is performed.

In FIG. 3L, a contact mask (not shown) is applied to open contactopenings through the BPSG layer 250 and the LTO layer 245 followed bydepositing a metal layer 260 after the contact mask (not shown) isremoved. The source polysilicon 220 as that shown in FIG. 2C, is simplyconfigured to directly contact through the continuous bottom-shieldingelectrode 130 as that provided in the macro-cell configuration thatincludes active cells and source connecting trench area shown in FIGS.2A and 2C. With the macro-cell configuration, the bottom-shieldingelectrode 220 is connected to the source metal 260 through theinterconnected trenched source connecting electrode 220 as shown. Thewafer is then followed the rest of standard trench MOSFET steps tocomplete the processing.

In another preferred embodiment as shown in FIGS. 3M-3O, the MOSFET hasa gate runner trench for connecting the trenched gate to the gate padfor outside connection. The gate runner trench 209-G as shown in FIG. 3Mmay have the same SGT structure and be formed during the same process asthe other SGT trenches, except that, the gate runner trench 209-G may bewider and deeper than other SGT trenches. Gate runner 209-G may also belocated in the termination area. Following the process described inFIGS. 3A-3L, in FIG. 3M, a metal mask (not shown) is applied to patternthe metal layer into gate metal 260-G and source metal 260-S. In FIG.3N, a plasma enhanced oxide and nitride deposition is carried out toform the oxide layer 270 and the nitride layer 280 as passivationlayers. Then in FIG. 3O, a passivation mask (not shown) is applied toetch the passivation layer to cover the gate metal to protect the gatemetal 260-G and to prevent a short circuit between the gate metal 260-Gand the source metal 260-S. The wafer is also thinned and back metal isdeposited to form the drain electrode.

According to above drawings and descriptions, this invention furtherdiscloses a power MOSFET device with a macro cell layout wherein eachmacro cell includes a plurality of polygon-shaped active cells whereineach cell comprising a source region separated by a plurality ofinterconnected trenches with a bottom-shielding electrode insulated fromand disposed below a trenched gate wherein the bottom-shieldingelectrode being electrically connected to at least a trenched sourceconnecting electrode formed within a section of the trenches dedicatedas a source connecting trench for connecting the bottom-shieldingelectrode to a source metal. In a preferred embodiment, the sourceconnecting trench extruding above the trenches for establishing a secureand reliable electrical connection with the source metal. In anotherpreferred embodiment, the power MOSFET further includes an insulationprotective layer disposed on top of the power MOSFET device having aplurality of source openings on top of the source region and thesource-connecting trench provided for electrically connecting to thesource metal. In another preferred embodiment, the power MOSFET devicefurther includes an insulation protective layer disposed on top of thepower MOSFET device MOSFET a gate opening provided for electricallyconnecting a gate pad to the trenched gate. In another preferredembodiment, the gate opening is disposed directly above a gate runnertrench in a termination area of the power MOSFET device. In anotherpreferred embodiment, the gate runner trench is wider and deeper thanthe trench provided for forming the trenched gate therein. In anotherpreferred embodiment, the gate runner trench further includes a SGTstructure with the bottom-shielding electrode. In another preferredembodiment, the power MOSFET device further includes a deep body-dopantregion in a termination area to function as a guard ring orJunction-termination extension type termination. In another preferredembodiment, the deep body-dopant region is deeper than and encompassinga gate runner trench in the termination area.

FIG. 4 shows a MOSFET device with specially configured termination areafor operation as a device for higher voltage rating. For a high voltageoperation, the formation of the termination area usually requires awell-controlled placement of trenches filled with the source polysiliconand a thick oxide. FIG. 4 shows an alternate embodiment to this method.A deep p-region 198 is implanted and diffused at the outset of theprocess to form a guard ring or Junction-termination extension typetermination. The guard ring or junction-termination extension formedwith the p-region 198 surrounds the gate 150 in electrical contact withthe gate metal 195-G.

FIG. 5 shows an alternate trenched gate configuration formed with a morecomplex process with a tapered oxide structure in the bottom-shieldingelectrode 130′ of the shield gate trench (SGT) disposed below thetrenched gate 150. The first oxidation is carried out to the greatestdesired thickness. After the polysilicon deposition and poly etch to adesired depth, a wet etch is performed to etch the oxide to a thinneroxide layer thickness along the trench sidewalls. A second polysilicondeposition and etch back is performed to a desired depth. The aboveprocesses are applied several times to provide a tapered polysilicon SGTstructure 130′ as shown in FIG. 5. At the penultimate polysilicon etchstep, a mask is applied to leave the polysilicon flush with the topsurface in the center of the source contact. Thereafter, the process isthe same as the one shown above. Another approach to form such a gradedoxide on the trench sidewall is to create a grading in implanted damagefrom a neutral species such as oxygen. Performing multiple implants intothe sidewall at different angles provides the grading in implanteddamage. The vertical implant has the highest dose for maximum damage. Asthe angle is increased, the dose is reduced to reduce the damage. Next,a single wet oxidation produces a tapered oxide profile along thesidewall. The advantage of the tapered oxide thickness is to allow theuse of a flatter epitaxial doping profile. The doping profile is easierto control to achieve the same Rds performance.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. For example, other conductivematerial instead of polysilicon may be used to fill the trenches.Accordingly, it is intended that the appended claims be interpreted ascovering all alterations and modifications as fall within the truespirit and scope of the invention.

We claim:
 1. A semiconductor power device comprising a plurality ofpower transistor cells surrounded by a trench opened in an active areaon a semiconductor substrate wherein: at least one of said cellsconstituting an active cell having a source region disposed next to atrenched gate electrically connecting to a gate pad and surrounding saidcell wherein said trenched gate further having a bottom-shieldingelectrode filled with a gate material disposed below and insulated fromsaid trenched gate; and said bottom-shielding electrode filled with saidgate material having a stepwise tapered profile having a stepwisereduced width and a substantially equal step height toward a bottom ofsaid trench with a lining layer surrounded said gate material having acorrespondingly stepwise increased thickness.
 2. The semiconductor powerdevice of claim 1 wherein: at least one of said cells constituting asource contacting cell surrounded by said trench with a portionfunctioning as a source connecting trench filled with said gate materialfor electrically connecting between said bottom-shielding electrode anda source metal disposed directly on top of said source contact trench.3. The trenched power semiconductor power device of claim 2 furthercomprising: an insulation protective layer disposed on top of saidsemiconductor power device having a plurality of source openings openedthrough the insulation protective layer on top of said source region andsaid source connecting trench provided for electrically connecting thesource region and the source connecting trench to said source metaldisposed on top of the insulation protective layer.
 4. The semiconductorpower device of claim 2 further comprising: an insulation protectivelayer disposed on top of said semiconductor power device having a gateopening opened through the insulation protective layer provided forelectrically connecting the trenched gate to said gate pad disposed ontop of the insulation protective layer.
 5. The semiconductor powerdevice of claim 4 wherein: said gate opening is opened through theinsulation protective layer directly above a gate runner trench in atermination area of said semiconductor power device.
 6. Thesemiconductor power device of claim 5 wherein said gate runner trench inthe termination area is wider and deeper than said trench surroundingthe transistor cells with said trenched gate disposed therein.
 7. Thesemiconductor power device of claim 5 wherein: said gate runner trenchfurther includes SGT structure with said bottom-shielding electrode. 8.The semiconductor power device of claim 1 further comprising: a deepbody-dopant region in a termination area to function as a guard ring orJunction-termination extension type termination.
 9. The semiconductorpower device of claim 8 wherein: said deep body-dopant region is deeperthan and encompassing said gate runner trench in said termination area.10. The semiconductor power device of claim 1 further comprising: adrain electrode disposed on a bottom surface of said semiconductorsubstrate.
 11. The semiconductor power device of claim 1 wherein: eachof said cells further includes a body region disposed between saidtrench surrounding said cells.
 12. The semiconductor power device ofclaim 1 wherein each of said cells further includes a body regiondisposed between said trench surrounding said cells wherein said bodyregion in said active cell encompassing said source region disposed nextto said trenched gate.
 13. The semiconductor power device of claim 1wherein: an oxide layer is disposed above said bottom-shieldingelectrode to shield and insulate said bottom-shielding electrode fromsaid trenched gate.
 14. The semiconductor power device of claim 1wherein: an oxide layer is disposed above said bottom-shieldingelectrode to shield and insulate said bottom-shielding electrode fromsaid trenched gate wherein said oxide layer above said bottom-shieldingelectrode is disposed in said trench with a predefined depth controlledby a timed etch process.
 15. The semiconductor power device of claim 14wherein: said semiconductor power device having a reduced gate to draincapacitance Cgd depending on a depth of said oxide layer above saidbottom-shielding electrode.
 16. The semiconductor power device of claim1 wherein: said power transistor cells further comprising trenched metaloxide semiconductor field effect transistor (MOSFET) cells.
 17. Thesemiconductor power device of claim 1 where: said bottom-shieldingelectrode having at least two stepwise reduced widths and substantiallyequal step height toward a bottom of said trench with a lining layersurrounded said gate material having at least two correspondinglygradually stepwise increased thicknesses.
 18. A semiconductor powerdevice comprising: a trenched gate having a bottom-shielding electrodefilled with a gate material disposed below and insulated from a trenchedgate wherein said bottom-shielding electrode filled with said gatematerial having a stepwise tapered profile having a stepwise reducedwidth and a substantially equal step height toward a bottom of saidtrench with a lining layer surrounded said gate material having acorrespondingly gradually stepwise increased thickness.
 19. Thesemiconductor power device of claim 18 where: said bottom-shieldingelectrode having at least two stepwise reduced widths and substantiallyequal step height toward a bottom of said trench with a lining layersurrounded said gate material having at least two correspondinglygradually stepwise increased thicknesses.